High-strength brass alloys are conventionally used for sliding members such as bearings, and first to fourth class high-strength brass castings specified in Japanese Industrial Standards JIS H5120 have been used. These high-strength brass alloys are made by adding Al, Fe, Mn, etc., to Cu—Zn alloys, whereby seawater resistance, toughness, wear resistance, and hardness are improved. Therefore, these high-strength brass alloys are widely used as sliding members such as synchronizer rings for transmission systems of automobiles, gear wheels for general machinery, and bearings.
The high-strength brass alloy has a matrix that may exhibit various phases such as a α phase, β phase, α+β phase, and γ phase, according to the total amount of added elements which are weighed by zinc equivalent (hereinafter called “amount corresponding to zinc equivalent”). If the amount corresponding to the zinc equivalent is small, the matrix exhibits the α phase. A high-strength brass alloy exhibiting the α phase has superior toughness but has low hardness. Therefore, when this high-strength brass alloy is used for sliding members, abrasive wear occurs easily. If the amount corresponding to the zinc equivalent is increased, the matrix exhibits the β phase. Moreover, if the amount corresponding to the zinc equivalent is further increased, the matrix exhibits the γ phase. In a high-strength brass alloy exhibiting the γ phase, hardness is increased and wear resistance is improved, whereas toughness is greatly decreased and shock resistance is decreased.
Accordingly, a high-strength brass alloy having a matrix made of a single structure of the β phase is widely used for sliding members because toughness is not decreased and wear resistance is superior. Nevertheless, in accordance with recent tendency to increase efficiency and prolong the life of mechanical devices, further improvement of wear resistance is required for sliding members made of high-strength brass alloy.
In order to improve wear resistance of a high-strength brass alloy having a matrix made of a α+β phase or a β phase structure, the following high-strength brass alloys are suggested. Japanese Examined Patent Publication No. 51-041569 discloses a high-strength brass alloy in which intermetallic compounds of manganese silicide system such as Mn5Si3 are dispersed in a matrix. In addition, Japanese Examined Patent Publications Nos. 62-057700 and 2-038651 disclose high-strength brass alloys in which Fe—Mn—Si intermetallic compounds are dispersed in a matrix.
It is known that wear resistance can be improved by dispersing the intermetallic compounds of a manganese silicide system or a Fe—Mn—Si system in a matrix. In this case, the zinc equivalent of Si is 10 and is very high in the elements added in a high-strength brass alloy. Therefore, the amount corresponding to the zinc equivalent is increased by adding Si. Accordingly, in order to maintain the matrix to be a single structure of the β phase, the amounts of the other elements should be limited. As one of the other elements, for example, Al is known. Al is an element for improving corrosion resistance and strengthening the matrix. Therefore, by dispersing Fe—Mn—Si intermetallic compounds in the matrix that is hardened by Al, wear resistance is further improved. However, the zinc equivalent of Al is 6 and is very high. If Si and Al are added, the amount corresponding to the zinc equivalent is increased, and the γ phase is generated in the matrix. As a result, wear resistance is improved, but elongation is greatly decreased. Therefore, when Si is added so as to improve wear resistance, the amount of Al must be decreased. Accordingly, it is difficult to further improve wear resistance while the matrix is maintained to be a single structure of the β phase.